The present invention relates to computed tomography (CT) imaging apparatus; and more particularly, to the use of contrast agents with such apparatus.
In a contemporary computed tomography system, an x-ray source projects a fan-shaped beam which is collimated to lie within an X-Y plane of a Cartesian coordinate system, termed the "imaging plane." The x-ray beam passes through the object being imaged, such as a medical patient, and impinges upon an array of radiation detectors. The intensity of the transmitted radiation is dependent upon the attenuation of the x-ray beam by the object and each detector produces a separate electrical signal that is a measurement of the beam attenuation. The attenuation measurements from all the detectors are acquired separately to produce the transmission profile.
The source and detector array in a conventional CT system are rotated on a gantry within the imaging plane and around the object so that the angle at which the x-ray beam intersects the object constantly changes. A group of x-ray attenuation measurements from the detector array at a given angle is referred to as a "view" and a "scan" of the object comprises a set of views made at different angular orientations during one revolution of the x-ray source and detector. In a 2D scan, data is processed to construct an image that corresponds to a two dimensional slice taken through the object. The prevailing method for reconstructing an image from 2D data is referred to in the art as the filtered backprojection technique. This process converts the attenuation measurements from a scan into integers called "CT numbers" or "Hounsfield units", which are used to control the brightness of a corresponding pixel on a cathode ray tube display.
The clinical value of an x-ray image resides in the contrast produced by differences in x-ray attenuation. Such differences are quite pronounced in some cases, such as bone versus surrounding soft tissues, but in others, the attenuation difference between adjacent tissues is very small. To enhance image contrast, a contrast agent may be intravenously administered to the patient prior to the scan. In dynamic liver studies, for example, the contrast agent helps to differentiate abnormal tissue from the surrounding normal tissue by increasing their x-ray attenuation difference. The resulting enhancement, when a contrast agent is injected, is a dynamic process, and the timing of the image scanning relative to the timing of this enhancement is critical to the diagnostic value of the study.
When a contrast agent is injected in a patient during a liver study, for example, the x-ray contrast goes through three circulation phases. The initial "bolus" phase lasts about 60 to 90 seconds while the contrast agent is being injected. In a dynamic liver study for example, vascular attenuation increases and is much greater than hepatic parenchymal attenuation in this phase. The next phase, called "non-equilibrium", lasts for another 60 to 90 seconds. During this phase, vascular attenuation decreases until it equals hepatic parenchymal attenuation. The "equilibrium" phase is reached once the vascular and parenchymal attenuations can no longer be differentiated on the basis of their x-ray attenuation characteristic.
Dynamic CT exams take advantage of the non-equilibrium phase by acquiring x-ray attenuation data for images when the attenuation differences are at a maximum. In a liver study, for example, during the non-equilibrium phase the hypovascular hepatic metastases appear darker than the surrounding parenchyma in an x-ray image. Since the non-equilibrium phase is short, it is important to start the exam at the optimal moment. Doctors and technologists rely on past experience and general medical knowledge to know when optimal contrast enhancement will be achieved in a given patient. They must take many things into consideration, like the patient's weight and hydration state and the patient's cardiac output. Other variables include contrast agent concentration and its rate of injection. Some clinical sites routinely perform "trial" contrast injection exams on patients before the prescribed diagnostic exam just to determine when optimal contrast enhancement will be achieved in the target tissues of those patients.
In some studies, such as vascular and perivascular studies, the use of contrast agents takes advantage of the bolus phase. Optimal contrast enhancement will happen very early after contrast agent injection begins (6-50 seconds after injection depending on the anatomy under study). If the exam is started too early, the first images will not have enough enhancement. If started too late, surrounding tissue will already be highlighted and can not be distinguished from the vascular structures under study.
Studies using contrast agents are often less than optimal because the scans miss the optimal contrast window for the type of study being performed. Doctors need a tool which will enable them to see when the optimal contrast level has been reached in the patient so that the exam can be started when it will produce the best images possible.